1. Field of the Invention
The present invention relates to techniques for encoding data on buses for the purpose of reducing the power dissipated on optical buses in telecommunications systems and has been developed with particular attention paid to its possible application to on-chip integrated buses, in particular buses of medium and small dimensions.
It is, in any case, to be borne in mind that the scope of the invention is more general. The invention is, in fact, applicable to all telecommunications systems in which there occur conditions of operation of the same type as the ones described in what follows.
2. Description of the Related Art
In modern telecommunications systems, intensive use has been made of optical fibers as physical medium of the transmission channel. In fact, optical fibers ensure a high frequency of the carrier and, by virtue of the wide band associated thereto, enable multiplexing of a large number of communication channels on a single fiber.
Optical-fiber telecommunications systems are particularly simple and involve sources for modulation of the optical signal on the fiber, such as lasers or LEDs and photodetection devices, which currently are mostly made up of discrete components but which, in the future, will be integrated on-chip.
However, said systems present certain technological limits. One of said limits lies in the need to carry out electro-optical and opto-electrical conversions for enabling processing of the signal. These conversions limit the transmission bandwidth. Furthermore, the attenuation introduced by the fiber on the signal and other noise sources make demodulation at the receiver end a problem of stochastic detection.
The signals received on an electrical bus are multiplexed on an optical fiber, by serializing the signal of the electrical bus and sending it over the individual optical fiber via modulation of an on-off-keying (OOK) type of the source.
FIG. 1 represents a system for transmission on a synchronous optical bus.
The system in question comprises a transmitter, designated as a whole by the reference 10, in which there is present a serializer device designated by 12, which receives from an electrical bus 11 the input signal, carries out a parallel-to-serial conversion thereon and supplies it to a driving device represented by the block 14 and designed to drive a laser diode 15 so as to cause it to emit on an optical fiber 16, which embodies the so-called optical bus. At the other end of the optical fiber 16 is a receiver 20, comprising a photodetector 21, for opto-electrical conversion of the received optical signal, followed by an amplifier 22 and a comparator 23, downstream of which is set a serial-to-parallel converter 24 that supplies the electric output signal.
From the serializer device 12 a clock signal CK is further obtained, which is transmitted by a laser diode 15′, driven by a corresponding driving device 14′, on an optical fiber 16′, is received at the receiver 20 by a photodetector 21′, followed by an amplifier 22′ and a comparator 23′, and is supplied to the serial-to-parallel converter 24, so as to drive correctly the operation of conversion.
In fact, since the data on the electrical bus 11 are transmitted on an integer number n of lines, in the parallel-to-serial conversion these data are converted to a frequency n times the frequency of the clock signal associated with the electrical bus 11. Hence, the clock signal CK is transmitted on the optical fiber 16′ in order to enable recovery of the data in reception and to solve problems of synchronization in detection.
The modulation adopted for the laser diode 15 is, as has been said, of the OOK type.
FIG. 2 shows the optical power P0(t) incident on the photodetector 21 as a function of time t, whilst FIG. 3 shows the current I(t) in the photodetector 21 as a function of time (t). The logic levels of the input signal are zero and one.
The modulated optical signal P0(t) is:
                                          P            0                    ⁡                      (            t            )                          =                              P            M                    ·                                    ∑              k                        ⁢                                                  ⁢                                          b                k                            ·                              p                ⁡                                  (                                      t                    -                    kT                                    )                                                                                        (        1        )            where PM indicates the power emitted by the laser source, bk a binary coefficient and p(t) the envelope of the signal. Hence, it is clearly a base-band pulse-amplitude modulation (PAM), where the elementary impulse response is a rectangular impulse.